I guess the composition doesn't feel quite right to me. The machine is off-center, but it's not balanced by something large on the left which makes the image seem off-balance.

Both abseilers line up with the cyclinders hanging from the roof which is also an unusual choice, unless there's a reason for it.

It's also a picture with a lot of bright areas spread almost evenly around the image. That's not something you'd usually go for in a composition. It could work in the final image, but it's hard to tell if it will from this sketch. It doesn't work at the moment IMO.

If you remove the top half of the image and treat the bottom half as a stand-alone composition then it's actually quite nice. The lower half has a good feel and the compositional balance (for shapes and values) is good. It's that top half that causes trouble._________________Art Links Archive -- Artists and Tutorials

You have the large foreground object (the guy) on the same side of the image as the large background object (the reactor), so it feels even more unbalanced now.

The the values composition still doesn't look right. Perhaps if the top half was mostly dark except for the portals or something. It's all too bright IMO._________________Art Links Archive -- Artists and Tutorials

I like the blue green colour-shift.
I agree with Sumaleth, the composition in your updated one feels unbalanced, i feel that the position of the abseilers should be swapped, i think that a foreground guy hanging to our right of the reactor would balance things out. My opinion anyways

Only thing I really dislike on the first pic, is that the men look like giants (compared to the rail) I realize theyre closer then the rail, but its just hard to tell due to shading and pose of the left guy (almost looks as if hes standing on it).